1. Field of the Invention
The present invention relates to a technique for evaluating a range in which a defect is present on the surface of a metal subject to be inspected or the length of the defect by use of a distribution of output voltages obtained by performing eddy current testing.
2. Description of the Related Art
The eddy current testing is performed as follows. That is, an alternating current is made to flow in an exciting coil, and the exciting coil is placed near the surface of a metal subject to be inspected. Then, an eddy current is induced in the subject. The eddy current varies due to a discontinuous portion (uneven portion) of the structure of the subject, which is a defect or the like (for example, a crack which is open on the surface of the subject to be inspected) present on the subject. A magnetic field, which depends on the eddy current, also varies according to the variation of the eddy current. Induced power generated in a detecting coil also varies due to the variation of the magnetic field. Based on the variation of the induced power, a defect present in the subject is detected.
Non-Patent Document 1 (Nishimizu, Koike, Matsui, Development of flexible multi-ECT sensor, Collected Writings on 8th Surface Detection Symposium Lecture (2005), pp 139-142) discloses an example of using a method for the eddy current testing to detect a defect present on the surface of a subject. In addition, Non-Patent Document 2 (Kawata, Kawase, Kurokawa, Intelligent ECT System (New type ECT system for inspection of heat transfer tube for steam generator), June Issue of Inspection Technology (2005), pp 66-72) discloses an example of using a method for the eddy current testing to detect a defect present on the surface of and the inside of a thin wall tube.
Since there is no specification or standard for evaluating the length of a defect found by the eddy current testing, the length of the defect has been evaluated by use of a combination of a scale that has been calibrated and visual inspection or liquid penetrant inspection.